1. Field of the Invention
The present invention relates to an active acoustic control apparatus.
2. Description of the Related Art
Generally, a driving motor for an electric vehicle is quieter than an engine. Such a feature is advantageous in terms of noise, but it is inferior to the engine from the viewpoint of notifying the presence of the vehicle to pedestrians in the surroundings. Taking into account the above-described situation, an electric vehicle is developed which generates an alert sound from an external loudspeaker (i.e., a loudspeaker mounted as an external component to a vehicle body) while the vehicle is moving (see, for example, Japanese Unexamined Patent Application Publication No. 05-208636). In the electric vehicle disclosed in Japanese Unexamined Patent Application Publication No. 05-208636, the alert sound transferred to the inside of the vehicle is canceled by outputting, from an internal loudspeaker, a cancellation sound to cancel the alert sound, which is in opposite phase to that of the alert sound output from the external loudspeaker. The alert sound is generated in accordance with a reference signal that is in sync with rotation of a driving motor (see, for example, “Abstract” of Japanese Unexamined Patent Application Publication No. 05-208636). Further, when the cancellation sound is generated, adapted control is performed by using both the reference signal and an inverse transfer characteristic that is inverse to a transfer characteristic of an in-vehicle space transfer system (see, for example, “Abstract”, paragraph [0021], and FIG. 3 of Japanese Unexamined Patent Application Publication No. 05-208636). The term “transfer characteristic of an in-vehicle space transfer system” used herein implies the transfer characteristic from the internal loudspeaker to a microphone (see, for example, paragraphs [0010], [0013], and [0022] and FIG. 3 of Japanese Unexamined Patent Application Publication No. 05-208636).
In the electric vehicle disclosed in Japanese Unexamined Patent Application Publication No. 05-208636, as described above, the alert sound is generated from the external loudspeaker in accordance with the reference signal that is in sync with the rotation of the driving motor, while the cancellation sound is output from the internal loudspeaker through the adapted control using both the reference signal and the inverse transfer characteristic that is inverse to the transfer characteristic of the in-vehicle space transfer system. Thus, the cancellation sound reflects the rotation of the driving motor and the transfer characteristic of the in-vehicle space transfer system. In other words, Japanese Unexamined Patent Application Publication No. 05-208636 does not take into account a transfer characteristic from the external loudspeaker to the microphone.
Depending on, for example, whether a vehicle window is opened or closed, however, the transfer characteristic from the external loudspeaker to the microphone is changed, thus causing a risk that an effect of canceling a sound, which is to be controlled, by the cancellation sound cannot be sufficiently developed unless the transfer characteristic from the external loudspeaker to the microphone is also taken into consideration.